1. Field of the Invention
The present invention relates to an aluminum nitride sintered body, and more particularly, it relates to an aluminum nitride sintered body allowing low-temperature sintering and having high strength and high thermal conductivity. The invention further relates to a method of preparing the same.
2. Description of the Prior Art
Aluminum nitride (AIN) having high thermal conductivity and a low thermal expansion coefficient is recently employed as the material for insulated substrates for various types of electronic components, in place of alumina which has been employed in general.
In general, however, aluminum nitride has a relatively high sintering temperature of at least 1800.degree. C. An existing sintering furnace or jig component cannot sufficiently withstand such a high temperature but must be frequently repaired or discarded/exchanged. Further, aluminum nitride sintered at a high temperature requires high sintering energy. Therefore, the cost for an aluminum nitride sintered body is higher than that for an alumina sintered body, to which hinders popularization of aluminum nitride.
In order to sinter aluminum nitride which is generally hard to sinter as compared with alumina, a sintering aid of an alkaline earth metal element compound or a rare earth element compound is mainly employed. Particularly in order to lower the sintering temperature, more specifically to enable sintering at a temperature of not more than 1700.degree. C., combined use of an alkaline earth metal element compound and a rare earth element compound is studied. Typically, various studies have been made on sintering aids prepared by combining calcium compounds and yttrium compounds.
For example, Japanese Patent Laying-Open No. 61-117160 (1986) describes an aluminum nitride sintered body obtained by normal pressure sintering under a temperature of not more than 1700.degree. C. with a sintering aid prepared by combining an alkaline earth metal element compound such as CaCO.sub.3 and a rare earth element compound such as La.sub.2 O.sub.3. Japanese Patent Laying-Open No. 63-190761 (1988) describes a sintering aid for aluminum nitride prepared by combining CaO and Y.sub.2 O.sub.3.
A technique of reducing an aluminum oxide contained in a sintered body with carbon or a material liberating carbon for improving the thermal conductivity of an aluminum nitride sintered body is generally known. For example, each of Japanese Patent Publication Nos. 7-5372 to 7-5376 (1995) discloses a method of increasing the thermal conductivity of aluminum nitride by nitriding an oxide contained therein through free carbon with a sintering aid of an yttrium compound. Further, Japanese Patent Laying-Open No. 58-55377 (1983) describes a method of reducing/removing oxygen by employing an alkaline metal compound as a sintering aid and adding carbon powder or the like.
In addition, it is known that a thick metallized film having high strength can be formed by introducing a rare earth element or an alkaline earth metal element into an aluminum nitride sintered body. For example, Japanese Patent Publication No. 5-76795 (1993) discloses a circuit board obtained by forming a conductor part or a dielectric part prepared from at least either paste containing Ag or paste containing Au on an aluminum nitride sintered body containing at least one element selected from a rare earth element and an alkaline earth metal element. Japanese Patent Publication No. 7-38491 (1995) describes a method of forming a conductive layer of a high melting point metal such as tungsten or molybdenum on an aluminum nitride sintered body containing at least one element selected from a rare earth element and an alkaline earth metal element.
As described above, sintering of aluminum nitride under a low temperature of not more than 1700.degree. C. has been enabled due to development of a new sintering aid prepared by combining an alkaline earth metal element compound and a rare earth element compound. Thus, the thermal conductivity of an aluminum nitride sintered body is improved, and such an aluminum nitride sintered body is increasingly applied to a substrate for an exothermic semiconductor element such as a power device.
In the aforementioned method employing the sintering aid of a rare earth element and/or an alkaline earth metal element, however, a rare earth aluminum oxide, an alkaline earth aluminum oxide, a rare earth alkaline earth aluminum oxide and the like are formed between an oxide present in the aluminum nitride sintered body and the sintering aid. Although formation of these oxides is necessary for the aforementioned low-temperature sintering under a temperature of not more than 1700.degree. C., the grain sizes of the sintered body are increased due to the oxides.
In recent years, aluminum nitride is frequently applied to a radiating substrate for a power module or a jig for semiconductor equipment, which is used under a strict heat cycle. Therefore, aluminum nitride must be improved in thermal shock resistance as well as strength for serving as ceramic. In this regard, the mean grain size of the aluminum nitride sintered body must be not more than 3 .mu.m, preferably not more than 2 .mu.m. In the conventional method, however, further improvement of the strength of the sintered body cannot be attained due to increase of the grain sizes resulting from formation of a large amount of oxides.